Igneous Rocks II
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Igneous rocks are born in fire. There are two types of igneous rocks. The first type and most common is the intrusive igneous rocks. These rocks form when a pocket of magma slowly cools down enough to form into solid rock.
The first thing that we need to know about rocks is: rocks are created from minerals. When a liquid cools to a solid, the substance is said to have crystallized. This means as magma cools, the elements in the magma form solid bonds with their neighbors in a repetitive pattern. The minerals form into bigger and bigger crystals until they smash into other crystals. Once enough mineral crystals smash together so hard that they can't be taken apart, the minerals have formed into an intrusive igneous rock. A great example of this type of rock is granite.
Intrusive igneous rock has coarse grained crystals. That means, if you look closely at the rock you can see the mineral crystals. If you look closely at a piece of granite you will see little flecks of white, black, gray, and sometimes pink. These flecks of color are crystals. If you can see the crystals that form an igneous rock, you're looking at an intrusive igneous rock.
The second type of igneous rock is called extrusive igneous rock. When magma reaches the surface of the earth people call it lava. Extrusive igneous rocks are formed when lava cools and forms into solid rock. This cooling is much faster than the slow cooling that forms intrusive igneous rocks. Extrusive igneous rocks have small grained crystals. Yes, they have crystals; they're just so small you can't see them. There are many different types of extrusive igneous rocks. The type of rock you get depends on what kind of lava the rock was formed from, and how fast the rock cooled.
Magma erupts to the surface becoming lava. The place that magma erupts to the surface is called a volcano. There are three main kinds of volcanoes and each produces their own kind of extrusive igneous rock. If the lava was formed at a high temperature and has a lower amount of silicon (the main ingredient of the mineral quartz), the lava will flow relatively smoothly and cool to form the rock basalt. The type of volcano that produces this kind of lava is called a shield volcano. The volcanoes in Hawaii are shield volcanoes and most of the rock they produce is basalt. When shield volcanoes erupt they normally don't explode. The lava flows up from inside the planet and pours down the side of the volcano like a thick hot syrup. These lava flows cover a large area and are very destructive to property. However, these volcanoes are less dangerous to people because the lava flows are usually very slow.
A Hawaiian shield volcano.
Thus, scientists divide igneous rocks into two different categories. Those which cooled below the surface, and those that cooled on the surface. Those which began their lives below the surface are called intrusive rocks, while those which cooled on the surface are referred to as extrusive rocks.